1. Field of the Invention
This invention is directed to forming improved load-bearing apertures in thermoplastic sheet material, and more particularly to journal-pin receiving apertures for hinge structures integrally incorporated in relatively thin thermoplastic sheet material. Increase of the bearing area in the aperture is provided over that afforded by the original thickness of the sheet material. The invention has particular application to an improved hinge joint for blow-molded containers of thermoplastic sheet material incorporating separate base and cover members hinged together along their confronting edges, or for connection of "suitcase-type" handles to such containers, or for any similarly hinged joint in thermoplastic sheet material where increased load-bearing capacity is needed. Typical containers of the type mentioned include those used for storing and transporting articles such as tools, musical instruments, electronic equipment, etc., which require packaging for easier transportation and protection against damage but at the same time must be readily accessible. Hinged blow-molded containers fill this requirement excellently, but prior hinge connections used in them have not been fully satisfactory.
2. Description of the Prior Art
Blow-molded plastic containers comprised of a base and a hinged cover have been developed and used extensively for the aforesaid purposes. A common form of container is comprised of a base and a cover or lid member joined by an integral web, forming what is commonly known as a "living hinge." This arrangement provides certain economies of molding, since both the base and lid can be molded in one operation with the integral web forming the hinge between them. However, molding considerations of forming a "living hinge" often restrict the designs of a container. Also, due to the "memory" effect of the plastic in a living hinge, it is not possible to position the container elements in any position other than full open or full closed without auxiliary means. Finally, the living hinge in normal (closed) position is under strain and is subject to cracking after an extended period of time, or where the hinge is exposed to heat or a chemical stress-cracking agent. For that reason, molded base and lid members having separable, cooperating hinge elements which, when joined together, permit unrestricted hinging movement, are often preferred. Each container member in this instance carries one of two elements of a hinge set. For example, one element may comprise a journal pin while the other comprises a complementary bearing aperture in a container wall. One example of such container is described in U.S. Pat. No. 4,005,800, the disclosure of which is incorporated herein by reference. Where, as in the embodiment illustrated in that patent, the container is of the double-wall type, one set of hinge elements consists of a journal pin or trunnion integrally molded in the cover or lid, for example, while the cooperating hinge element consists of a receiving aperture drilled through the inner wall of the base in a side wall flanking the hinged edge of the container. In the prior construction the journal pins bear simply on the thickness of the wall, which is not very great at such points, so the strength of the hinge joint is accordingly limited. Heavy unit loads imposed on the bearing areas involved in those prior hinge constructions cause distortion of the aperture by cold (i.e., plastic) flow, thus altering the size and shape of the aperture. Such a condition is also prevalent where a hinge formation is employed in providing a "suitcase" handle for a container, in addition to the hinged cover and base combination mentioned above. The result of the cold flow is a weakened hinge joint, leading to accidental disengagement of the hinge elements, and separation of the carton base and lid, or dropping of the container in case of a defective handle hinge.
Improvement in the hinge strength would result if the wall thickness of the container could be increased in molding the bearing area for the hinge pins. However it is not readily possible, or at least not practical, in blow-molding to increase the thickness of the wall only at selected points of the container without increasing the thickness throughout other wall areas as well. This is not only unnecessary for strength considerations in such other areas, but is generally wasteful of material and adds appreciably to the weight and expense of the container.